The present invention relates to a filter for electric signals, having a substrate, a body that vibrates relative to the substrate, and electrodes connected to a signal input and/or output for electric excitation and/or detection of vibration of the body.
The filter effect of the mechanical transmission response of a weekly attenuated vibrating structure has long been known. Mainly surface wave filters are used today in electronic communications processing. This type of filter is usually made of a piezoelectric material at whose surface are mounted electrodes that intermesh like combs, producing at the surface electric fields with polarities that alternate in the longitudinal direction of the comb structures. These fields induce a piezoelectric deformation of the material, which thus induces surface waves at a suitable frequency of a voltage applied between the electrodes. Since these filters use a piezoelectric substrate material, they are not suitable for monolithic integration into semiconductor structures.
Therefore, work has already been underway for several years on micromechanical structures that can be produced from a semiconductor material and, when operated as resonators, have an electromechanical filter response. Such a filter is described, for example, by Kun Wang, Yinglei Yu, Ark-Chew Wong and Clark T. C. Nguyen, xe2x80x9cFree-Free Beam High-Q Micromechanical Resonators.xe2x80x9d
One problem with these known micromechanical resonators is the parasitic overcoupling, i.e., signal paths created by parasitic capacitance between the signal input and output of the resonator, through which the input signal crosses over directly to the filter output and is thus overlaid on the filter response, sometimes massively.
To counteract this problem, filters with a compensating capacitance or differential capacitor arrangements with downstream formation of the difference have been developed. The effect of these arrangements is based on the fact that a signal with a 180xc2x0 phase shift is passed through a capacitance similar to the parasitic capacitance to thereby obtain a signal simulating the parasitic signal but with the opposite polarity; this signal can then be superimposed additively on the filter output signal to eliminate the effect of the parasitic capacitance. For example, a dummy resonator structure can be constructed for this purpose to simulate the electric response of the resonator except for the resonance.
In the case of a differential capacitor arrangement, an electrode is mounted above and below the vibrating body, for example. When the body swings upward, the distance from the upper electrode becomes smaller and thus the capacitance between the upper electrode and the vibrating body becomes greater, while at the same time the capacitance between the lower electrode and the body becomes smaller. With a suitable design, the parasitic capacitances for both electrodes will be the same. By forming the difference between the signals from the upper and lower electrodes, the influence of parasitic capacitance can be largely eliminated.
Neither approach for eliminating the problem of parasitic capacitance is completely satisfactory. When using compensating capacitances, for example, knowledge of the exact parasitic capacitance and its frequency response allows construction of a suitable dummy resonator. This involves extremely complicated development work, and a dummy resonator structure developed for a given resonator structure cannot readily be applied to a newly designed resonator.
Using the differential capacitance requires that electrodes be mounted in different planes above and below the vibrating body. Such a design is simpler to develop, but the need for electrodes arranged at different levels makes production of such a resonator more complicated and expensive.
The filter according to the present invention for electric signals has the advantage of being insensitive to parasitic capacitances, requiring little development effort and being inexpensive to manufacture in comparison with the known technology.
The idea on which the present invention is based is that the electrodes for detecting the vibration which are assigned to antipodes that are deflected in phase opposition are connected to two separate terminals of the signal output. This makes it possible to mount each electrode on the substrate on one side of the vibrating body while nevertheless varying the capacitance between the electrodes and the vibrating body in phase opposition.
Vibration of the movable body in question here is antisymmetrical, i.e., changing polarity as the body moves to either side of an axis of symmetry, and the electrodes are arranged symmetrically with respect to this plane of symmetry of vibration. This guarantees that the amplitude of motion of the vibrating body will be the same in the area of the electrodes.
In the simplest case, which is the preferred case, the vibration is the first harmonic of the fundamental mode of the bending vibration of the body.
According to a preferred further embodiment of the filter, a control electrode is mounted in allocation to an area between two antipodes of the vibration, and a control potential can be applied to this control electrode for tuning the resonant frequency of the body. With the help of such an electrode, the resonant frequency of the filter, which would otherwise be predetermined in a fixed manner by the manufacturer, can be adapted to prevailing needs. With the help of such a control electrode, the resonant frequency of the vibrating body can be manipulated even after manufacturing the filter; for example, a filter installed in a circuit can be fine tuned with the help of a potential applied to the control electrode.
Because of the higher symmetry in excitation and detection of the vibration, it is preferable to have an electrode connected to the signal input arranged in the area of one antipode of the vibrating body to excite the vibration and an electrode connected to the terminal of the signal output to detect the vibration.
As an alternative, one electrode may be arranged in the area of each antipode, connectable by time division multiplexing to a terminal of the signal input for exciting the vibration and to a terminal of the signal output for detecting the vibration. This design results in a highly effective capacitive decoupling of the chronological separation of excitation and detection of the vibration.
In one embodiment of the filter which is preferred because of its simplicity, the movable body includes at least one bar fixedly connected to the substrate at its longitudinal ends.
As an alternative, the vibrating body may have at least one bar connected to the substrate by arms acting on nodes of vibration.
Especially good decoupling of input and output signals can also be achieved by providing the electrodes for excitation on one of these partial bodies and the electrodes for detecting the vibration on the other partial body. The resulting spatial separation of excitation and detection electrodes effectively limits capacitive signal overcoupling from the excitation electrode to the detection electrode.
The two partial bodies are preferably mechanically connected by an arm arranged in the area of one node of vibration. This connection offers an important advantage. It acts on the partial body at the center between the excitation electrodes, i.e., exactly at the point where the voltage drop is compensated by material-induced ohmic resistance in the partial body and also where electrostatic leakage fields are mutually minimized, thus also minimizing a charge flow which might be induced between the partial bodies due to the excitation, so that maximum suppression of the overcoupling from the excitation electrodes to the detection electrodes by way of the vibrating body is achieved.
The coupling between the two partial bodies is preferably not rigid, but instead the two are loosely connected. Thus with the help of a control electrode which can receive a control potential and is mounted on each partial body in an area between two antipodes of the vibration, it is possible to tune the resonant frequencies of the partial bodies independently within certain limits. This results in an increase in resonant width of the entire filter. In other words, not only the position but also the width of the feed-through window of the filter can be influenced by the applied control voltage. This effect can also be additionally utilized to compensate for temperature-induced attenuation properties of the vibratory motion which affect filter quality.
Since the filter according to the present invention may be made of a semiconductor material, circuits for pre- and/or postprocessing of the signal to be filtered can be integrated into the same substrate in an expedient manner.